1. Technical Field
The present disclosure relates to systems and methods for controlling evaporative emissions in vehicles having an internal combustion engine that may have limited operating cycles and/or may be operated with low manifold vacuum, including hybrid electric vehicles (HEV's).
2. Background Art
Vehicles having internal combustion engines including hybrid electric vehicles incorporate various strategies for managing fuel vapors that may be generated during refueling or when resting (with the engine off) due to temperature and pressure variations within the fuel tank, for example. Evaporative emission control systems and methods may use one or more canisters that capture and temporarily store fuel vapors to reduce or prevent the vapors from escaping to atmosphere. The canisters are periodically purged of stored fuel vapors during engine operation using vacuum created by a throttle valve in the intake manifold to route the vapors to the engine cylinders for combustion in combination with liquid fuel from the fuel tank. Canister purging cycles are controlled to mange engine performance, evaporative emissions, and exhaust emissions while minimizing any perceptible change in engine/vehicle operation. The length of time required to purge the canister(s) depends on various operating parameters including the amount or level of available vacuum generated within the intake that draws fresh air through the canister(s) to purge the stored fuel vapor. Engine and vehicle operating constraints may also impact the available or acceptable purge rate and thereby impact the time required for a complete purge of the canister(s).
Various engine/vehicle technologies have been developed to improve overall fuel efficiency that impact the control of evaporative emission control systems. One approach is to reduce engine pumping losses by reducing manifold vacuum. This may be accomplished by eliminating the throttle valve and using other airflow control devices, such as in some variable cam timing or variable valve timing applications, for example. Where a throttle valve is employed, operating the engine with the throttle valve position closer to wide open whenever possible also lowers intake manifold vacuum and reduces pumping losses. Representative engine technologies that limit engine operation and/or operate with low manifold vacuum under more operating conditions include variable cam timing, variable valve timing, gasoline turbocharged direct injection, and engines used in hybrid electric vehicles, for example.
Hybrid electric vehicles combine an internal combustion engine in various configurations with an electric motor/generator and one or more batteries to power the vehicle. The internal combustion engine may be used when needed to power the motor/generator to recharge the batteries and/or to power the vehicle in combination with the battery. Most strategies attempt to minimize operation of the internal combustion engine and to operate the engine unthrottled or near wide-open throttle for better fuel efficiency. However, purging or regenerating the vapor storage canister(s) requires running the internal combustion engine to draw the vapors into the engine cylinders and provide combustion of the vapors.
Commonly owned U.S. Pat. No. 6,557,534 discloses a canister purge strategy during idling for a hybrid electric vehicle having a single vapor canister that may selectively increase intake manifold vacuum by electronically controlling the throttle valve to increase the purging rate during a purging cycle. The engine speed is controlled by the electric motor to prevent engine stumbling or stalling otherwise associated with rapid ingestion of the fuel vapor. Commonly owned U.S. Pat. No. 5,111,795 discloses an integrated evaporative emission system with integrated or dedicated primary and refueling canisters connected in parallel to capture fuel vapors. A fluidic controller is used to route the vapors to the canisters with one or more purge valves used to route the vapor to the engine during purging. While suitable for many applications, neither strategy is generally applicable to various types of engines having limited operation and/or low intake manifold vacuum.